It has been established in horizontal continuous casting that a casting is intermittently drawn to allow for the shrinkage thereof upon cooling.
As shown in FIGS. 8 and 9, a typical existing horizontal continuous casting equipment comprises a tundish 50 for receiving molten metal 51. The molten metal 51 is fed through a tundish nozzle 52 and a breakring 53 into a copper mold 54 cooled by a water-cooling jacket 55 and intermittently drawn out as a casting 56 under solidification by an intermittent drawing device (not shown) as indicated by the arrow A. Indicated at 56a is a solidified shell.
In such a casting apparatus, the intermittent withdrawal by the drawing device of the casting 56 is conducted according to the drawing mode shown in FIG. 10. More particularly, the casting is first drawn at a predetermined speed by a predetermined amount (line portion I). Next, after some retention time (line portion II) the casting is pushed back a predetermined length (line portion III). Finally, the casting is stopped (line portion IV) until the next withdrawal starts. These steps constitute one cycle of the intermittent drawing operation.
Now the phenomena which occur in the mold 54 during a single cycle of the drawing operation will be described with reference to FIGS. 11a-11h. FIG. 11a shows the state of the solid shell 56a just on the verge of starting the withdrawal. In this state, the trailing end surface 56b of the solid shell 56a addheres to the front end surface 53a of the breakring 53. Since even the most thin shell part has an enough thickness, the shell end surface 56b can be separated from the ring end surface 53a by subsequent withdrawal without tearing at an intermediate portion of the solid shell 56a. As a result, the molten metal flows into the gap between the shell end surface 56b and the ring end surface 53a to form a secondary solidified shell 56c which is extremely thin at the middle thereof and thick adjacent both of the end surfaces 53a and 56b (FIG. 11b). The secondary shell 56c grows in length as the parent (primary) shell 56a advances. The secondary shell 56c cannot strongly bond to the end surface (actually interface) 56b of the parent shell 56a since the parent shell end surface 56b has already crystalized to an increased extent by the previous contact with the breaking end surface 53a and thus differs in crystalline structure from the secondary shell 56c. This interface 56b is called "cycle mark" by those skilled in the art and is known to cause subsequent crack formation. The secondary shell 56c provides at the position contacting the breakring end surface 53a a new end surface 56d which will become another cycle mark later.
The length growth speed of the secondary shell 56c is generally smaller than the advancing speed (drawing speed) of the parent shell 56a. Accordingly, further advance of the parent shell 56a from the position shown in FIG. 11b ultimately results in the breakage of the secondary shell 56c at the thin middle portion thereof to form divided secondary shells 56c' (FIG. 11c). The molten metal immediately flows into the gap between both of the divided secondary shells 56c' to form a tertiary shell 56e (FIG. 11d).
In the same way, further advance of the parent shell 56a from the position shown in FIG. 11d causes the tertiary shell 56e to break into divided tertiary shells 56e' (FIG. 11e) and to thereby form a fourth shell 56f between both of the divided tertiary shells 56e' (FIG. 11f).
One cycle of the casting drawing operation is completed by the above described steps. The shell crystalline structure changes discontinuously across the interfaces between the shell segments 56c', 56e', 56f formed during the processes illustrated in FIGS. 11c to 11f. Though being not so serious as the aforementioned cycle mark, these interfaces constitute defects called "tear marks" since they lead to subsequent formation of depressions and wrinkles. Thus, the tear marks together with the cycle marks deteriorate the quality of the casting.
The shell state after the push-back stroke is illustrated in FIG. 11g in which all shell segments formed between the parent shell 56 and the breaking 53 are generally represented as one piece by reference numeral 56a' for convenience although the tear marks are actually present as shown in FIG. 11f. As will be seen from FIG. 11g, after the push-back action the interposed shell 56a' has increased in thickness in correspondence with the amount of the push-back stroke and the degree of crystalization (solidification) of the molten metal during the push-back period.
In the subsequent pause period, further crystallization of the molten metal allows the interventing shell 56a' to grow to such a thickness (FIG. 11h) that the intervening shell 56a' can withstand the tensile force required to separate the now shell end face 56d from the breakring end face 53a upon withdrawal of the next drawing cycle. This state is substantially identical to the state shown in FIG. 11a.
By repeating the above described intermittent drawing cycle (drawing, push-back and pause) a casting is generally continuously produced, and in so doing a multiplicity of cycle marks (one for each cycle) and tear marks are formed on the opposite surfaces of the casting 56 (FIG. 8) as hereinbefore described.
One way to avoid defects in crystal structure caused by cycle marks and tear marks for improved casting quality is to increase the cycle speed of the drawing operation. It is for the following reason that high cycle intermittent drawing operation eliminates crystal structure defects associated with cycle marks. In high cycle intermittent drawing operation, the parent shell end face 56b contacts the breakring end face 53a for a reduced time, so that crystalization at the shell end face 56b proceeds to a decreased extent. As a result, discontinuity in shell crystal structure across the interface 56b between the parent shell 56a and the secondary shell 56c is not so serious as to render the cycle mark (which per se does not disappear) defective. Further, since high cycle intermittent drawing leads to a shorter withdrawal stroke, tearing of the secondary shell 56c, i.e., formation of tear marks, is unlikely to occur.
In the case of producing a casting having a large cross section, however, the large weight of the casting requires an increased energy for intermittent withdrawal. In addition, it is naturally required to install behind the intermittent drawing device a cutting machine for cutting the casting at constant spacing, the mass of the cutter is added to the casting through the blade of the cutter engaging the casting during cutting operation. As a result, it is required to design the drawing machine in consideration of the inertia attributable to the combined mass of the casting per se and the cutting machine. Thus, in case a casting of a large thickness is desired, limitations are imposed on realizing high cycle intermittent drawing operation to ensure sufficiently improved product quality.
Instead of employing an intermittent drawing machine, Published Unexamined Japanese Patent Application No. 54-82328 discloses a continuous casting method in which a tundish provided fixedly with a mold and a breakring is horizontally vibrated by a vibrator while a casting is continuously withdrawn at a constant speed by a continuous drawing device (The tundish in this case is similar to the one illustrated in FIG. 8 but additionally has casters to impart horizontal movability.). According to this method, when the tundish is moved by the vibrator in a direction opposite to the casting drawing direction, the net (relative) speed of the casting withdrawal from the mold becomes higher than the drawing speed afforded solely by the continuous drawing device. On the other hand, when the tundish is moved in the casting drawing direction, the mold moves faster than the casting being moved by the drawing device, so that the casting is pushed back relative to the mold. Thus, the tundish vibrating type casting machine of this prior art is also accompanied by cycle marks and tear marks as in the intermittent drawing type casting machine described hereinbefore, and the problems attendant with the cycle marks and the tear marks can be similarly eliminated or relieved by increasing the frequency of the oscillating tundish.
However, the weight of the tundish receiving molten metal is so heavy that it is extremely difficult to vibrate the tundish at a high frequency. Further, weight variation due to a decrease in the surface level of the molten metal within the tundish makes it difficult to suitably control the amplitude and frequency of the oscillating tundish.